Animals make a wide variety of sounds, which include vocalizations—bird calls, dog barks, etc.—but also other sounds, for example sounds made using body parts other than vocal chords and sounds produced when the animal is engaged in some activity (e.g. knocking with a beak, eating, digging, etc.).
Ethologists, farmers, zoo keepers, pet owners, and the like have long known that many animal sounds are meaningful and relate to the animal's behaviour, situation, state of mind and/or identity (gender, age, size and, also, individual identity). Numerous different experiments have now shown that there can be a relationship between an animal's behavioural context (situation/motivational state) and/or identity and measurable acoustic properties of sounds that the animal makes.
Experiments have shown that the acoustic properties of vocalizations produced by birds, various mammals (primates, deer, dogs, elephants, . . . ), anurans and other land-and sea-based animals can vary dependent on the behavioural context (desire to mate, presence of a predator, etc.) and/or on the size, gender and age of the animals (this latter property making it possible, in some cases, to recognize an individual animal based on its vocalizations).
For example, in relation to the effect of behavioural context, the Centre for Mammal Vocal Communication Research at the University of Sussex has recently found that domestic cats can produce a particular type of purr, including an embedded high-frequency component (similar to a human baby's cry), when they are trying to solicit attention or food from their owners. Moreover, the present inventors were part of a research team which analyzed barks made by Hungarian sheepdogs in seven different behavioural contexts and found that a machine-based acoustic analysis of the barks could often successfully determine the behavioural context in which a particular bark was produced (see “Classification of dog barks: a machine learning approach” by Molnar et al, published in Animal Cognition, 11(3), pages 389-400, Springer Verlag, 2008).
In addition to these “meaningful vocalizations”, some of the other sounds made by animals, either deliberately or incidentally as they go about their daily lives, can be interesting because they can help to indicate what is the animal's current behaviour, state of mind, etc.
Animals also react to sounds that they hear, notably by changing the nature or intensity of their behaviour. For example, if a domestic animal is hungry and hears the sound of its owner approaching, it may move to stand by its feeding dish. As another example, if a domestic or zoo animal is frightened and then it hears the voice of a human it knows well, it may calm down. As yet another example, seals are known to behave differently depending on whether sounds they hear from an approaching whale are sounds made by a killer whale (predator) or sounds made by another, non-predator species.
In some experiments, ethologists play synthesized or recorded animal calls to one or more animals in order to observe how these animals will react to the sounds. However, these experiments are essentially manual processes, using machinery merely for the generation and outputting of sounds and recording of animal audio responses. Moreover, these experiments can be described as “one-shot” processes, investigating the immediate effect that a given sound produces on animal behaviour. Furthermore, in these experiments the animals are reacting to human stimulus, the tests do not examine interactions where the animal creates the initial stimulus.